White and yellow traffic markings used for demarcating traffic lanes is a common sight on almost all roads. These markings ensure safe driving conditions under varying weather conditions. As described in U.S. Pat. Nos. 3,337,483 and 3,523,029, these white and yellow markings are formed from thermoplastic marking compositions which contain a hydrocarbon resin, fillers, a white or yellow pigment or dye, and glass beads. These marking compositions are applied to a surfaces, such as a road, as a hot melt Upon application, the marking compositions form a layer on the surface of the pavement which is allowed to cool to ambient temperature so that a solidified layer is formed within a short period of time. The compositions are generally applied to the pavement of roads, parking lots, sidewalks and runways. Other exemplary traffic marking compositions are described in U.S. Pat. Nos. 3,897,378; 3,998,645; 4,025,476; 4,324,711; 4,406,706; 5,213,439; and 5,709,908.
At the present time large amounts of thermoplastic marking compositions are used for yellow traffic marking purposes. The conventional coloring component which has generally been incorporated in the yellow thermoplastic traffic paints, is lead chromate or cadmium yellow pigment alone or mixed with coated chrome yellow pigment. These pigments are used in large quantities, for example, two pounds of lead chromate are used per gallon of marking composition. Unfortunately, these pigments contain the lead, cadmium and chromium heavy metals which are toxic to the environment. Thus, while desirable from a color point of view, these heavy metal pigments are discouraged due to their toxic nature. Indeed, with the increasing environmental concern surrounding the use of heavy metal pigments in paints and coatings, there is a push to replace heavy metals in hot melt marking compositions with organic pigments or dyes. Many states, such as California and Texas, have already mandated that lead chromate is to be replaced where possible. Consequently, efforts are being made to utilize other colorant materials; ie., nontoxic pigments and dyes, for yellow traffic marking paint purposes.
The efforts to replace the heavy metal pigments have not been successful as severe requirements are imposed on the performance of the thermoplastic marling compositions, and consequently on the property of colorants which they contain. For example, a fairly high resistance to heat is required, because the thermoplastic marking compositions are applied after they are melted by heating. Moreover, a weathering resistance which is durable for more than one year is also very essential as the thermoplastic marking composition are typically used outdoors.
Previous attempts to formulate hot melt marking compositions containing organic dyes have not met with success. For example, both U.S. Pat. No. 3,337,483 and U.S. Pat. No. 3,523,029 describe hot melt marking compositions containing a coloring agent, such as Benzidine Yellow and Benzidine Orange. Unfortunately, Benzidine colorants and their intermediates are highly toxic and are suspected to be carcinogenic. Thus, there exists a need in the art for a hot melt marking composition which is non-toxic and safe to the environment.